Aiding magnets for minimizing length of reversal zone



A. H. IVERSON Filed June 17, 1963 s ,x mm N J 7 M 4 WM m W 5 E \9\ m:

AIDING MAGNETS FOR MINIMIZING LENGTH OF REVERSAL ZONE REVERSA June 27, 1967 ATTORNEY United States Patent 3,328,619 AIDING MAGNETS FOR MINIMIZING LENGTH OF REVERSAL ZONE Arthur H. Iverson, Redwood City, Calif., assignor to General Electric Company, a corporation of New York Filed June 17, 1963, er. No. 288,273 6 Claims. (Cl. 313-84) The invention relates to a magnet system for focusing beams of charged particles. More particularly the invention relates to a magnet system for producing a magnetic field which rapidly reverses direction at least once along a predetermined path of charged particle flow and wherein the axial magnetic field along the path can be made to approximate a square wave. The invention is particularly useful for providing a magnet system of relatively light weight for focusing the electron beam of a low-noise traveling Wave tube.

It is now well-known that the weight of a beam focusing magnet system can be substantially reduced by the use of a periodic magnetic field as compared to a uniform field. Such periodic magnetic fields have been discussed, for example, by J. T. Mendel et al. in an article entitled, Electron Beam Focusing With Periodic Permanent Magnet Fields, Proceedings of I.R.E., volume 42, pp. 800-810, May 1954. An advantage attendant the reduction in weight of the magnet system is the reduction of fringing fields which allows close spacing between tubes or between a tube assembly and other magnetic material without the need of bulky and heavy magnetic shielding.

In typical periodic focusing arrangements a relatively short magnetic period is provided and the axial magnetic field approximates a sinusoid. A short magnetic period minimizes the ripple in the electron beam diameter and allows minimization of size and weight of the focusing structure. However, short magnetic period structures give rise to difficulties in a traveling wave tube, for example, 'in introducing R.F. wave energy to the slow wave structure without undue disturbances of the magnetic field which result in undesirable perturbations of the electron beam. Thus it has been found that the noise figure of traveling wave tubes focused by short period periodic magnetic fields is in the order of 3 db higher than tubes .focused with uniform fields.

It has been found that by using an alternating magnetic field having a longer magnetic period a significant weight advantage over a uniform field structure can be achieved while providing a noise figure comparable to a uniform field if the alternating magnetic field is properly shaped. More particularly, it is desirable that the magnetic field change direction in a minimum distance to thereby reduce perturbing effects on the electron beam. It is known in the art that shaping of the magnetic field can be achieved by shunting the magnetic fiux intermediate of the reversal zones. However this technique is obviously wasteful-of magnetic power. Furthermore, such an approach does not significantly reduce the length of the reversal zone. It is desirable to provide an alternating magnetic field for focusing an electron beam of a traveling wave tube whereby the weight and bulk of the magnet system are reduced while at the same time achieving a relatively low noise figure.

It is therefore a primary object of the invention to reduce the disturbing effect of magnetic focusing field reversal on a magnetically focused electron beam.

It is another object of the invention to provide an improved magnet system for focusing an electron beam.

It is a further object of the invention to minimize the length of the reversal zone of an alternating magnetic field.

It is another object of the invention to provide auxiliary means for shaping an alternating magnetic field.

It is another object of the invention to provide a low noise traveling wave tube in a system wherein the electron beam is focused by a magnetic field which alternates in direction along the axis of the electron beam.

These and other objects of the invention are achieved by providing a magnet structure comprising a plurality of main magnets placed end-to-end along the path of the electron beam to be focused, with adjacent ends of the main magnets being of like magnetic polarity whereby the magnetic field reverses direction at each junction of the main magnets. To appropriately shape the axial mag netic field, a pair of auxiliary magnets is positioned at each junction of the main magnets, each in aiding relationship to the respectively associated main magnet. These auxiliary magnets serve to reinforce the axial magnetic fields of the main magnets in the region of their adjacent ends whereby the composite axial magnet field is made to approximate a square wave in shape. Alternatively, by increasing the strength of the auxiliary magnets, the magnetic field may be peaked in the region of the main magnet junction. In either case the length of the reversal zone is reduced with a consequent reduction in the disturbance of the electron beam.

A preferred embodiment of the invention is described more specifically with reference to the accompanying drawings in which:

FIGURE 1 illustrates a traveling wave tube incorporating a magnetic focusing structure (shown in longitudinal section view) according to the present invention;

FIGURE 2 illustrates the variation of the axial magnetic field along the electron beampath due to the main magnets alone;

FIGURE 3 illustrates the composite axial magnetic field of the main magnets and the auxiliary magnets along the electron beam path; and

FIGURE 4 illustrates the peaking of the magnetic field adjacent the reversal zone by the use of auxiliary magnets of greater strength.

Illustrated in FIG. 1 is a traveling wave tube 10 of Wellknown type which is adapted to employ the magnet system of the present invention for focusing its electron beam along the tube axis.

The various elements of the tube are enclosed in an evacuated envelope 11 which may be formed of a material such as glass. The various elements of the tube include an electron gun or cathode 12 which when heated (by means not shown) provides the electrons for the electron beam. Positioned at the opposite end of the tube is a collector electrode 13. The collector electrode 13 is maintained (by means not shown) at a high potential relative to the cathode 12. Thus the cathode 12 and the collector electrode 13 define a longitudinal path of electron flow along the axis of the tube.

The tube 10 further includes a plurality of electrodes 14, 15 and 16 to which appropriate potentials are applied for introducing the electrons from the cathode 12 into a drift tube 17 and thence through a slow-wave structure, illustrated in FIG. 1 as a helix 18, to the collector electrode 13. An input coupler 19' is provided for introducing R.F. energy to the helix 18 and an output coupler 20 is provided for extracting R.F. energy from the helix. The input R.F. field maybe applied to the input coupler through a coaxial line 21. Similarly, the RF. energy may be received from the output coupler 20 by means of a coaxial line 22. Further features and details of operation of traveling wave tubes of the type illustrated are well known and will not be repeated therein, the present invention being directed to an improved magnet system for focusing the electron beam.

To achieve eflicient and low noise operation of a traveling wave tube it is important to minimize discontinuities and disturbances of the magnetic field which can introduce ripple of the electron beam. According to the present invention, a magnet system for achieving these results comprises a plurality of main magnets, illustrated in FIG. 1 as a pair of cylindrical permanent magnets 23 and 24, a plurality of pole pieces, illustrated in FIG. 1 as a first annular end pole piece 25, a second annular end pole piece 26 and an annular reversal pole piece 27, and a pair of auxiliary magnets associated with each reversal pole piece, illustrated in FIG. 1 as a pair of permanent ring magnets 28 and 29.

The pole pieces 25, 26 and 27 are formed of magnetically permeable material such as soft iron, Permalloy, or the like, and they are positioned at the ends and between the main magnets to direct the magnetic flux therefrom to form a longitudinal field along the axis of the traveling wave tube. The thickness of the reversal pole piece 27 should be minimized, consistent with avoiding magnetic saturation thereof, as its thickness influences the length of the reversal zone which it is the object of the invention to minimize.

As illustrated in FIG. 1, the main magnets 23 and 24 are placed end-to-end along the path of the electron beam to be focused with adjacent ends of the main magnets being of like magnetic polarity whereby the axial magnetic field reverses direction in the region of the reversal pole piece 27 which separates the adjacent ends of the main magnets 23 and 24. This reversal of direction of the magnet field is shown in FIG. 2 which illustrates the variation of the magnetic field intensity B along the longitudinal axis X of the tube. The curve of FIG. 2 illustrates the axial field intensity due to the main magnets 23 and 24 alone, that is, without the auxiliary of aiding magnets 28 and 29, and it is noted that an undesirably long reversal zone obtains.

To decrease the length of the reversal zone whereby the deleterious effects of the magnetic field direction reversal is minimized, the auxiliary magnets 28 and 29 are placed adjacent the reversal pole piece 27 in the region of the reversal zone. The auxiliary magnets 28 and 29 are longitudinally magnetized and are oriented in aiding relationship to the associated main magnets. Thus auxiliary magnet 28 is oriented in the same direction as the associated main magnet 23. Similarly, the auxiliary magnet 29 is positioned in aiding relationship to main magnet 24.

The magnetic fields of the auxiliary magnets, thus positioned in the reversal region, serve to reinforce the axial magnetic fields of the main magnets in this region whereby a more rapid reversal of direction obtained and the length of the reversal zone greatly reduced. This substantial reduction in length of the reversal zone is shown in FIG. 3 which illustrates the composite axial magnetic field of the main and auxiliary magnets. (It is noted that the auxiliary magnets 28 and 29 should be formed of a material having a coercive force sufficient to resist demagnetization by the magnetic fields of the main magnets.)

By appropriate choice of dimensions and field strength of the auxiliary magnets in relation to the main magnets, the axial magnetic field may be arbitrarily shaped within wide limits. For example, for some application it may be desirable to peak the magnetic field adjacent the reversal zone as shown in FIG. 4. This and other shapes can be accomplished by proper choice of the auxiliary magnets through routine experimentation in accordance with the teaching of the present invention herein.

There has thus been described a magnet system of relatively light weight and having low fringing fields for producing an alternating beam focusing field wherein extended variation of the magnetic field is avoided whereby efiicient and low noise operation of a traveling wave tube can be achieved.

While the illustrated embodiment of the invention has been described as utilizing main magnets which are permanent magnets of cylindrical shape, this is not a limitation of the invention. Other equivalent main magnets 4 may be employed, for example, electromagnets or horseshoe shaped permanent magnets bridging the pole pieces can be used.

Similarly, while the illustrated embodiment of the invention utilizes aiding magnets in the form of permanent ring magnets, other forms of introducing an aiding magnetic field into the reversal region can be provided without departing from the principles of the present invention.

Furthermore, while the magnet system specifically described herein provides a single magnetic field reversal over the length of the electron beam to be focused, the principles of the present invention can obviously be applied to a magnet system providing a plurality of such field reversals.

Thus, while the principles of the invention have now been made clear in an illustrative embodiment there will be obvious to those skilled in the art many modifications in structure, arrangement, proportions, the elements, materials, and components used in the practice of the inevntion which are specifically adapted for specific environments and operating requirements, without departing from those principles. The appended claims are therefore intended to cover and embrace any such modifications, within the limits only of the true spirit and scope of the invention.

What is claimed is:

1. A magnet system for focusing an electron beam over an elongated path, comprising: a pair of hollow cylindrical main magnets surrounding said electron beam and positioned end-to-end along said path, adjacent ends of said main magnets being of like magnetic polarity whereby the axial magnetic field along said path reverses direction within a reversal zone in the region of said adjacent ends of said main magnets; a first ring magnet positioned in said region coaxially with and in aiding relationship to a first magnet of said pair of main magnets; and a second ring magnet positioned in said region coaxially with and in aiding relationship to the second magnet of said pair of main magnets; said first and second ring magnets each having magnetic fields separate from the axial magnetic field of the main magnet; the magnetic fields of said ring magnets being superimposed upon and aiding the magnetic field of the main magnets, the aiding fields of said first and second ring magnets serving to reduce the length of said reversal zone.

2. A magnetic system for focusing an electron beam over an elongated path, comprising: a pair of main magnets surrounding said electron beam and positioned endto-end with like poles adjacent along said path whereby the axial magnetic field along said path reverses in a reversal zone in the region of the adjacent poles of said main magnets; and means for minimizing the length of said reversal zone comprising a pair of aiding magnets surrounding said electron beam, one of each main magnet, said aiding magnets being positioned in the region of the adjacent poles of said main magnets, each aiding magnet producing a magnetic field separate from, and superimposed upon, the magnetic field of the respectively associated main magnet in said region.

3. A magnet system for producing a substantially uniform magnetic field in a given direction along a first portion of a predetermined path and for producing a substantially uniform magnetic field in a direction opposite said given direction along the remaining portion of said predetermined path, comprising: a first hollow cylindrical main magnet surrounding said first portion of said path; a second hollow cylindrical main magnet surrounding said remaining portion of said path, adjacent ends of said main magnets being of like magnetic polarity; a plurality of magnetically permeable pole pieces including a reversal pole piece between the adjacent poles of said :main magnets; a first ring magnet adjacent said reversal pole piece and positioned coaxially with and in aiding relation to said first main magnet; and a second ring magnet adjacent said reversal pole piece and positioned coaxially with and in aiding relationship to said second main magnet, the respective strengths of said first and second ring magnets being sufiicient to partially compensate for the decrease of the axial fields of said main magnets in the region of said reversal pole piece.

4. In a traveling wave tube having means for establishing a flow of electrons between an electron gun and an electron collector, a magnet system for maintaining said electrons in a beam, comprising: a plurality of magnetically permeable annular pole pieces axially aligned with said beam including a first pole piece at the gun end of said tube, a second pole piece at the collector end of said tube, and a reversal pole piece positioned intermediate of said first and second pole pieces; a first magnetic field producing device for producing an axial magnetic field in a given direction between said first and said reversal pole pieces; a second magnetic field producing device for producing an axial magnetic field in a direction opposite said given direction between second and said reversal pole pieces; and auxiliary magnetic field producing separate devices for producing local axial magnetic fields superimposed upon and aiding the magnetic fields of said first and second devices in the region of said reversal pole piece.

5. In a traveling wave tube having means for establishing a flow of electrons between an electron gun and an electron collector, a magnet system for producing a substantially uniform magnetic field along the path between said gun and said collector in a given direction over a first portion of said path and for producing a substantially uniform magnetic field along said path in a direction opposite said given direction over the remaining portion of said path for maintaining said electrons in a beam along said path, comprising: a first hollow cylindrical permanent main magnet surrounding said beam over said first portion of said path; a second hollow cylindrical permanent main magnet surounding said beam over said remaining portion of said path, said first and second main magnets being oppositely poled with respect to said path; a magnetically permeable pole piece separating adjacent ends of said main magnets; a first permanent ring magnet positioned adjacent said pole piece, positioned coaxially with said first main magnet and producing a separate axial magnetic field in aiding relation to the axial magnetic field of said first main magnet; and a second permanent ring magnet positioned adjacent said pole piece, positioned coaxially with said second main magnet and producing a separate axial magnetic field in aiding relation to the axial magnetic field of said second main magnet.

6. In a traveling wave tube having an electron gun at one end and an electron collector at the other end for establishing a flow of electrons therebetween, a magnet system for maintaining said electrons in a beam comprising: a plurality of magnetically permeable annular pole pieces axially aligned with said beam including a first end pole piece at the gun end of said tube, a second end pole piece at the collector end of said tube, and at least one reversal pole piece positioned intermediate of said end pole pieces; a plurality of main magnetic field producing devices for producing axial magnetic fields interacting with said beam between said pole pieces the direction of the magnetic field reversing at each reversal pole piece; and auxiliary magnetic field producing separate devices for producing local axial magnetic fields aiding the magnetic fields of said main magnetic field producing devices in the regions of said reversal pole pieces.

References Cited UNITED STATES PATENTS 2,305,884 12/1942 Litton 313-84 X 2,867,745 1/1959 Pierce 315-35 2,911,554 11/1959 Kompfner et al. 313-84 X 2,947,907 8/1960 Bodmer 315-35 3,013,173 12/1961 Sturrock 313-84 FOREIGN PATENTS 1,180,631 6/1959 France.

835,271 5/1960 Great Britain.

JAMES W. LAWRENCE, Primary Examiner. ROBERT SEGAL, Examiner. 

2. A MAGNETIC SYSTEM FOR FOCUSING AN ELECTRON BEAM OVER AN ELONGATED PATH, COMPRISING: A PAIR OF MAIN MAGNETS SURROUNDING SAID ELECTRON BEAM AND POSITIONED ENDTO-END WITH LIKE POLES ADJACENT ALONG SAID PATH WHEREBY THE AXIAL MAGNETIC FIELD ALONG SAID PATH REVERSES IN A REVERSAL ZONE IN THE REGION OF THE ADJACENT POLES OF SAID MAIN MAGNETS; AND MEANS FOR MINIMIZING THE LENGTH OF SAID REVERSAL ZONE COMPRISING A PAIR OF AIDING MAGNETS SURROUNDING SAID ELECTRON BEAM, ONE OF EACH MAIN MAGNET, SAID AIDING MAGNETS BEING POSITIONED IN THE REGION OF THE ADJACENT POLES OF SAID MAIN MAGNETS, EACH AIDING MAGNET PRODUCING A MAGNETIC FIELD SEPARATE FROM, AND SUPERIMPOSED UPON, THE MAGNETIC FIELD OF THE RESPECTIVELY ASSOCIATED MAIN MAGNET IN SAID REGION. 